# std::ranges::set_union, std::ranges::set_union_result

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< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
template< std::input_iterator I1, std::sentinel_for<I1> S1, std::input_iterator I2, std::sentinel_for<I2> S2, std::weakly_incrementable O, class Comp = ranges::less, class Proj1 = std::identity, class Proj2 = std::identity > requires std::mergeable<I1, I2, O, Comp, Proj1, Proj2> constexpr set_union_result<I1, I2, O> set_union( I1 first1, S1 last1, I2 first2, S2 last2, O result, Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {} ); | (1) | (since C++20)
template< ranges::input_range R1, ranges::input_range R2, std::weakly_incrementable O, class Comp = ranges::less, class Proj1 = std::identity, class Proj2 = std::identity > requires std::mergeable<ranges::iterator_t<R1>, ranges::iterator_t<R2>, O, Comp, Proj1, Proj2> constexpr set_union_result<ranges::borrowed_iterator_t<R1>, ranges::borrowed_iterator_t<R2>, O> set_union( R1&& r1, R2&& r2, O result, Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {} ); | (2) | (since C++20)
Helper types |  |
template< class I1, class I2, class O > using set_union_result = ranges::in_in_out_result<I1, I2, O>; | (3) | (since C++20)

Constructs a sorted union beginning at result consisting of the set of elements present in one or both sorted input ranges `[`first1`,` last1`)` and `[`first2`,` last2`)`.

If some element is found `m` times in `[`first1`,` last1`)` and `n` times in `[`first2`,` last2`)`, then all `m` elements will be copied from `[`first1`,` last1`)` to result, preserving order, and then exactly max(n - m, 0) elements will be copied from `[`first2`,` last2`)` to result, also preserving order.

The behavior is undefined if

- the input ranges are not sorted with respect to comp and proj1 or proj2, respectively, or
- the resulting range overlaps with either of the input ranges.

1) Elements are compared using the given binary comparison function comp.
2) Same as (1), but uses r1 as the first range and r2 as the second range, as if using ranges::begin(r1) as first1, ranges::end(r1) as last1, ranges::begin(r2) as first2, and ranges::end(r2) as last2.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

### Parameters

first1, last1 | - | iterator-sentinel pair denoting the first input sorted range
first2, last2 | - | iterator-sentinel pair denoting the second input sorted range
r1 | - | the first input sorted range
r2 | - | the second input sorted range
result | - | the beginning of the output range
comp | - | comparison to apply to the projected elements
proj1 | - | projection to apply to the elements in the first range
proj2 | - | projection to apply to the elements in the second range

### Return value

{last1, last2, result_last}, where result_last is the end of the constructed range.

### Complexity

At most \(\scriptsize 2\cdot(N_1+N_2)-1\)2·(N1+N2)-1 comparisons and applications of each projection, where \(\scriptsize N_1\)N1 and \(\scriptsize N_2\)N2 are ranges::distance(first1, last1) and ranges::distance(first2, last2), respectively.

### Notes

This algorithm performs a similar task as ranges::merge does. Both consume two sorted input ranges and produce a sorted output with elements from both inputs. The difference between these two algorithms is with handling values from both input ranges which compare equivalent (see notes on LessThanComparable). If any equivalent values appeared `n` times in the first range and `m` times in the second, ranges::merge would output all n+m occurrences whereas `ranges::set_union` would output std::max(n, m) ones only. So ranges::merge outputs exactly \(\scriptsize (N_1+N_2)\)(N1+N2) values and `ranges::set_union` may produce less.

### Possible implementation

    struct set_union_fn
    {
        template<std::input_iterator I1, std::sentinel_for<I1> S1,
                 std::input_iterator I2, std::sentinel_for<I2> S2,
                 std::weakly_incrementable O, class Comp = ranges::less,
                 class Proj1 = std::identity, class Proj2 = std::identity>
        requires std::mergeable<I1, I2, O, Comp, Proj1, Proj2>
        constexpr ranges::set_union_result<I1, I2, O>
            operator()(I1 first1, S1 last1, I2 first2, S2 last2,
                       O result, Comp comp = {},
                       Proj1 proj1 = {}, Proj2 proj2 = {}) const
        {
            for (; !(first1 == last1 or first2 == last2); ++result)
            {
                if (std::invoke(comp, std::invoke(proj1, *first1),
                                      std::invoke(proj2, *first2)))
                {
                    *result = *first1;
                    ++first1;
                }
                else if (std::invoke(comp, std::invoke(proj2, *first2),
                                           std::invoke(proj1, *first1)))
                {
                    *result = *first2;
                    ++first2;
                }
                else
                {
                    *result = *first1;
                    ++first1;
                    ++first2;
                }
            }
            auto res1 = ranges::copy(std::move(first1), std::move(last1), std::move(result));
            auto res2 = ranges::copy(std::move(first2), std::move(last2), std::move(res1.out));
            return {std::move(res1.in), std::move(res2.in), std::move(res2.out)};
        }

        template<ranges::input_range R1, ranges::input_range R2,
                 std::weakly_incrementable O, class Comp = ranges::less,
                 class Proj1 = std::identity, class Proj2 = std::identity>
        requires std::mergeable<ranges::iterator_t<R1>, ranges::iterator_t<R2>,
                                O, Comp, Proj1, Proj2>
        constexpr ranges::set_union_result<ranges::borrowed_iterator_t<R1>,
                                           ranges::borrowed_iterator_t<R2>, O>
            operator()(R1&& r1, R2&& r2, O result, Comp comp = {},
                       Proj1 proj1 = {}, Proj2 proj2 = {}) const
        {
            return (*this)(ranges::begin(r1), ranges::end(r1),
                           ranges::begin(r2), ranges::end(r2),
                           std::move(result), std::move(comp),
                           std::move(proj1), std::move(proj2));
        }
    };

    inline constexpr set_union_fn set_union {};

### Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <iterator>
    #include <vector>

    void print(const auto& in1, const auto& in2, auto first, auto last)
    {
        std::cout << "{ ";
        for (const auto& e : in1)
            std::cout << e << ' ';
        std::cout << "} ∪ { ";
        for (const auto& e : in2)
            std::cout << e << ' ';
        std::cout << "} =\n{ ";
        while (!(first == last))
            std::cout << *first++ << ' ';
        std::cout << "}\n\n";
    }

    int main()
    {
        std::vector<int> in1, in2, out;

        in1 = {1, 2, 3, 4, 5};
        in2 = {      3, 4, 5, 6, 7};
        out.resize(in1.size() + in2.size());
        const auto ret = std::ranges::set_union(in1, in2, out.begin());
        print(in1, in2, out.begin(), ret.out);

        in1 = {1, 2, 3, 4, 5, 5, 5};
        in2 = {      3, 4, 5, 6, 7};
        out.clear();
        out.reserve(in1.size() + in2.size());
        std::ranges::set_union(in1, in2, std::back_inserter(out));
        print(in1, in2, out.cbegin(), out.cend());
    }

Output:

    { 1 2 3 4 5 } ∪ { 3 4 5 6 7 } =
    { 1 2 3 4 5 6 7 }

    { 1 2 3 4 5 5 5 } ∪ { 3 4 5 6 7 } =
    { 1 2 3 4 5 5 5 6 7 }

### See also

ranges::set_difference (C++20) | computes the difference between two sets (algorithm function object)
ranges::set_intersection (C++20) | computes the intersection of two sets (algorithm function object)
ranges::set_symmetric_difference (C++20) | computes the symmetric difference between two sets (algorithm function object)
ranges::merge (C++20) | merges two sorted ranges (algorithm function object)
ranges::includes (C++20) | returns true if one sequence is a subsequence of another (algorithm function object)
set_union | computes the union of two sets (function template)

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